Fibre reinforced polymer compositions and process and apparatus for production thereof

ABSTRACT

Fibrous network structures are produced within liquid polymer resins by passing the fiber-containing resin along a channel having a plurality of sets of flow modifying elements which establish a regulated succession of velocity profiles for the principal flow direction and the two directions perpendicular thereto. The individual velocity profiles persist over distances which are small compared to the channel dimension over which they are established and are such that there is substantially no net deviation from the principal flow direction. The velocity profiles superimpose on each other to cause rotation and sliding of the fibers so that a coherent network structure is built up which persists through extrusion dies and molds into the solid state. By means of a large number of touches per fiber the structures thereby established confer efficient mechanical reinforcing properties and enhanced thermal properties on the polymer composition.

This invention relates to the manufacture of fibre reinforced polymerand prepolymer artefacts particularly (but not exclusively) thosearising from extrusion, injection moulding and preimpregnated sheet ormat preparation.

It has long been appreciated that the addition of glass or other stifffibres to a thermoplastic or thermoset in a suitable fashion usuallybrings increased stiffness and strength to the processed material. Inthe case of thermoplastics the glass fibre has until recently been shortoften in the range of 0.3-0.6 mm. In the case of thermoset compositionsthe fibres have either been long (c.25 mm) discrete fibres or continuousthrough a very considerable proportion of the artefact. If long discretefibres are used they are usually either constructed into a loose wovenmat and then impregnated with thermoset materials or scattered in arandom overlapping fashion on to a layer of polymer with further polymerpoured on top. In either case a form of semi-coherent fibre structure isobtained within the polymer liquid, this structure being maintainedafter the composite sets to solid. This coherent structure is one of themain reasons why fibre reinforced thermoset composites tend to showgreater strength and stiffness than do the thermoplastic varieties basedon shert fibres, which do not usually form such structures. Suchshort-fibre compositions have an advantage however in that they areprocessed in the same continuous or automatic ways which are used forthermoplastics on their own.

As will readily be appreciated the thermoset advantage carries with it aprocessing disadvantage by comparison with its thermoplastic competitorin that generally a good deal of semi-manual intervention is required.

Over the last few years polymer granules with relatively long (3-15 mm)glass fibres have become available for automatic processing,particularly by injection moulding. While these can show considerableadvantages over their short fibre (0.3-0.6 mm) counterparts in someapplications, the flow fields set up by the die or moulds to shape theartefacts exercise a major adventitious influence on the materialproperties of the finished artefact (as they do with short fibres). Inparticular, for flows with a predominant velocity component in onedirection as in extrusion and many mouldings, fibres tend to be disposedvery largely in that direction.

When reinforcement is required in all directions, as it usually is, suchparticular fibre orientations give rise to major weakness in theperpendicular direction.

Besides the objective (a) of increasing the strength and stiffness oforganic polymer matrices, fibre structures may also be required in someapplications to meet other objectives either singly or in combination.These include (b) improvements to the thermal conductivity of thecomposition so that for instance it may be cooled faster after shapingthereby permitting higher rates of production, (c) reduction of the netthermal expansion and contraction of an artefact subjected toenvironmental temperature changes, (d) with electrically conductingfibres, to permit the passage of appreciable electric current betweentwo points of the artefact at moderate potential difference so that forinstance parts of the artefact may be fused to other artefacts, (e)again with electrically conducting fibres to inhibit the passage ofelectromagnetic radiation through extended surfaces of an artefact (e.g.a panel or cylinder) in order to protect for instance an electronicsystem from interferences.

Hitherto such reinforcing structures (as distinct from the short-fibrecompositions which do not form such structures) have been constructedbefore being brought into contact with a liquid resin or molten polymer.This method has two broad disadvantages in that first the reinforcingstructure must be made to confirm to the artefact shape in a separatemanufacturing step and secondly special steps must be taken in theshaping process to ensure that fibres are in sufficient contact with theresin or polymer, i.e. the fibres are well wetted by the resin orpolymer. It is not only the fact that the structure is preformed whichinhibits wetting: the fact that as mentioned above the fibres areusually themselves bundles of monofilaments sets an additional obstacleto effective wetting.

In practice, wetting is generally achieved by pressing or sucking orspraying the polymer or resin into suitably thin layers of thereinforcing structure and then adding more layers until sufficientthickness in the artefact has been built up. With some combinations offibre construction and expensive high performance polymers one suchlayer may be sufficient however.

Such reinforcing methods inevitably produce a laminate construction inwhich there is small or zero degree of fibre contact from layer orlayer. On the other hand when in well-known technology short fibrefilaments (usually of lengths 0.3 to 0.6 mm) are mixed with polymer orresin prior to shaping the distribution of fibres may well besubstantially uniform through the artefact but no structure is obtainedbecause the average number of contacts per fibre is too small.

It is an object of the present invention to provide a process andapparatus for producing network structures in situ in a polymer resincontaining discrete fibres, as well as new polymer productsincorporating such structures.

In the following descriptions polymer resin, will be taken to includepolymer melts, prepolymer liquids, viscous liquids generally.

According to a first aspect of the present invention there is providedpolymer resin processing apparatus comprising a channel with a principalflow direction along which fibre containing resin may flow defined byfixed surfaces, said channel having sets of flow modifying elementsadapted to superimpose a regulated succession of velocity profiles onthe principal flow direction and on the two directions perpendicularthereto, the flow modifying elements being so dimensioned and orientatedand the sets being so positioned relatively to each other such that thevelocity profiles established by each such set only persist over adistance which is small compared to the flow direction dimension of thechannel and to the principal transverse dimension of the channel andsuch that resin entering the first set at a particular position in thecross-section of the channel exits from the last set at substantiallythe same position on the cross-section.

According to a second aspect of the invention there is provided aprocess for the preparation of a fibre filled liquid polymer compositioncomprising passing liquid resin containing discrete fibres along a flowchannel adapted to impose on the resin a regulated succession ofvelocity profiles on the principal flow direction and on the other twodirections perpendicular thereto, the individual profiles persistingover distances which are small compared to the principal flow directiondimension of the channel and to the principal transverse dimension ofthe channel, the profiles being such that resin at a particular positionon the cross-section of the channel where the first of the sequencevelocity profiles is imposed is at substantially the same position onthe cross-section of the channel immediately after the last of thesequence of velocity profiles has been superimposed and such that themutually perpendicular velocity profiles superimpose on each other so asto result in rotation and/or sliding of the fibres relative to the resinsuch that as the resin passes along the channel the fibres build up anetwork structure.

The invention is applicable particularly to the formation of fibrestructures in thermoplastic resins which are then extruded to form, forexample, sheet or pipe. It is however also within the scope of theinvention to form the fibrous structure in a thermoplastic resin whichis to be moulded, or in a thermosetting resin which is subsequentlycured.

The mutually perpendicular velocity profiles established in the resinsuperimpose on each other and cause the fibres to rotate and/or sliderelative to the resin and to each other by a degree which is dependenton the position and orientation of the fibre in the resin, passing fromone element to the nest. Preferably the velocity profiles establishedfor the flow direction and the width (or circumferential extent) of thechannel are more significant than those established for the depthdimension of the channel. The movement of the fibres cause the fibres tocriss-cross over each other with some of the fibres becoming inserted inthe interstices between overlapping fibres to form a broadly coherentinterlaced or semi-woven structure within the polymer resin. Thestructure so established in the fluid state may be deformed as a resultof fibres sliding relative to each other but otherwise persistsundisrupted after passing through or into shaping dies or moulds intothe solid state.

The flow modifying elements may be provided by projections, vanes orpassages whose dimensions are small compared with the lateral and flowdirection dimensions of the channel.

The invention rests on five concepts:

(a) Sufficiently long discrete fibres (e.g. 5-15 mm) can be made to movein specific ways relatively to a resin (rather than just following it)and to each other by the superposition of sequences of velocity profileson the basic flow (usually unidirectional) along a channel leading to,for example, forming dies or moulds.

(b) through choice of particular velocity profile sequences, thisrelative motion can be used to form semi-woven or lace-like structureswithin the fluid flow. These structures can deform withoutdisintegration under the influence of shaping dies or moulds downstreamof the said channel and they persist into the solid artefact. Theorientation of fibres within the structure can be largely controlled tomatch the mechanical and other demands placed on the solid artefact. Forexample, the fibres may be orientated transversely as well as along thechannel direction.

(c) as discussed more fully below, the coherence and deformability ofsuch structures depends on the number of near-touches (which term asused herein to mean approach of two fibres within a distance of lessthan one fibre diameter--including actual contact) which an averagefibre makes on other fibres. The near-touches act as slider-hinges inthe structure and allow the necessary adaptation to the flow in the diesand moulds. The average number (N) of such near-touches per fibre isgiven approximately by:

    N˜Ac(l/d)                                            (1)

where A depends on the structure-type and to some extent on the volumefraction of fibres but is in the range of 0.5 to 2.5 for the structureobtained with this invention `c` is the volume fraction of fibres, `l`their length and `d` their diameter. Poor structures will have A muchless than 0.5 and, in the limit, where the fibres form no structure atall (e.g. when they are all aligned in one direction) A is O.

(d) to both facilitate the forming of a structure on a sufficiently finescale and to avoid in fluids such as polymers, a distorting elasticmemory effect in the bulk flow (which would show up as a twist in amoulding or non-uniform swell in an extrudate for example), the sets ofindividual velocity profiles are arranged so that, averaged overdistances which are small compared with the lateral dimensions of thechannel, the mean flow is always in the channel direction (i.e. bulkcross flows are eliminated).

(e) it follows from (c) and (d) that for optimum effect, particularscale relationships connect the fibre concentration c, aspect ratio(l/d) and the design of flow modifying elements.

The flow modifying elements may be incorporated in the die passages inthe case of extrusion, or the mould runners in the case of injectionmoulding. The elements are designed and arranged to create a network,e.g. a lace-like or semi-woven structure within the resin more or lessuniformly across the width of a sheet, around the perimeter of a hollowsection or through the thickness of a solid section. A variety ofstructures may be obtained by different designs and dispositions of theelements. The elements work by causing a regulated sequence of velocitychanges which extend over lengths which are small compared with themaximum dimension of the extrudate section but which are of the sameorder as the fibres. These scale relationships are crucial to obtainingboth the type of structures required and the uniformity across a sectionon which a satisfactory product depends. The essential point is that thegross motion of the resin along the die is not disturbed on a scalelarge enough to impose a different flow history on any appreciable partof the extrudate from that pertaining to the extrudate section as awhole. This provides an important advantage over other systems, forinstance spiral flows obtained by static or rotating die core means, aswell as applying naturally to non-circular hollow and solid sections,which they do not.

Typical thermoplastic compositions include for instance nylons,polypropylene, polyethylene, copolymers of these, matrix modifyingmaterials such as EDPM rubbers, together with a proportion of fibrematerials typically glass fibres. Coherent fibre structures andsignificant benefits can be obtained with the volume of fibres lowerthan 1% of total volume: typically the volume of fibres might be in therange 1-8% though higher proportions may be employed.

Typical thermosetting compositions include for instance unsaturatedpolyester resins, fillers and other additives together with a proportionof fibre materials as for thermoplastics. As such both finishedmouldings and pre-impregnated sheets and other forms may be prepared,the fibre structures herein replacing existing forms of fibrousreinforcement e.g. chopped strand mat, woven rovings etc.

In addition to sequences of velocity profiles a sequence of viscosityprofiles through the resin may be employed to influence the fibrestructure. This is of importance where as in pipe extrusion the pressureand time of application of pressure available to obtain a good surfacefinish is necessarily limited. Thus within and between the flowmodifying elements and the shaping mould or die, one or more surfaces ofthe channel may be heated above the temperature of the preceding part ofthe channel to establish a viscosity profile in the resin at rightangles to the channel direction. The temperature differentialestablished in the resin may be 5°-25° C. This has the effect ofreducing the concentration of fibrous material in the immediate vicinityof the surface thereby improving the surface finish. Where increase ofthe fibre concentration near the surface is required, the reverseprocedures may be applied.

The reinforcement which may be incorporated in the polymer materials arefibres of discrete lengths (e.g. 5-15 mm) which are short compared withany appreciable length of extrudate; in particular they are shortcompared with the maximum dimension of a hollow section or the width ofa sheet, but they are not necessarily short compared with the wallthickness of a hollow section or with the thickness of a sheet. Withinthese constraints a further distinction as to fibre length is drawnwhich is important to the main applications of the invention. Shortfibres are defined as those which, at the preferred loadings in thepolymer, on average do not directly affect the motion of another fibreas the polymer composition moves through the processing machinery. Longfibres are those which on average influence by touching or coming nearerthan a fibre diameter one or more fibres.

A number of manufacturers supply polymer material for use in the processin the form of fibre filled thermoplastic granules.

A further aspect of the invention resides in new polymer products per sewhich may be obtained using the above described method and apparatus. Inaccordance with this aspect of the invention there is provided a fibrefilled polymer product comprising a polymer matrix containing a networkof discrete fibres in which the orientation of the fibres issubstantially independent of the axis along which resin has flowedduring manufacture of the product, the network being comprised of majorstrands of a plurality of filaments and in-fill of mainly singlefilaments in spaces bounded by the major strands wherein the averagenumber of near touches N (as herein defined) which each fibre makes withadjacent fibres is in the range 0.5×c (l/d) to 2.5×c (l/d) with aminimum value of 8, where c is the volume fraction of the fibres in theproduct and is in the range above 0.005 to 0.1 for a thermoplastic andin the range above 0.005 to 0.4 for a thermosetting resin, and l/d isthe aspect ratio of the fibres.

Preferably N is in the range 0.8×c (l/d) to 2.5×c (l/d).

In the case of a thermosetting resin, the viscosity thereof is muchlower than that of the thermoplastic melts and as such it is possible touse higher volume fractions of fibres (due to the fact that they aremore easily wetted). In such a case, the volume fraction may be up to0.4.

Preferably c is in the range 0.001 to 0.08, more preferably 0.005 to0.05. The fibres are preferably monofilaments and preferably have alength of 3 to 15 mm. In one advantageous embodiment of the invention,the fibres may be electrically conducting and the product is such thatthe network structure prevents the passage of electromagnetic wavesthrough the material. In another embodiment, the fibres may have atleast 10 times the conductivity of the polymer, and the networkstructure increases the thermal conductivity of the product by at leastone third×c×thermal conductivity of the fibres.

The term A in the above equation may be considered to represent a`structure efficiency`. A low value of A means an ineffective structure(e.g. fibres lying predominantly in one direction) so far as number oftouches are concerned. Prior art moulding and extrusion of fibre filledpolymers use comparatively high fibre volume fractions and produceadventitious structures which are comparatively inefficient. Thesignificance of the product of the invention is that a more efficientnetwork structure is obtained, at lower fibre concentrations, than inthe case of the adventitious structure obtained in the prior artparticularly is this so with extrusions where only very low values of Awill be obtained adventitiously; the structure of the product of theinvention can also comprise a much higher proportion of fibresorientated at right angles to the predominant extrusion or moulddirection than would be obtained adventitiously.

In the fibre structure, the fibres are not knotted or significantlytwisted together, but the interlacing or interweaving of the fibrestypically but not necessarily in the length range 3 to 15 mm, achievesthe required number of near-touches per fibre to permit a degree ofrelative movement without disruption between different parts of thestructure in both the liquid and solid states of a resin. The structurescan be produced without the use of specific fibre matrix couplingagents.

When the average number of near-touches exceeds a certain number whichwill vary somewhat with the particular construction employed but istypically of the order of eight to thirty (e.g. ten to twenty) arecognisable coherent three dimensional structure can be set up bycontrol of the motions within a polymer resin which is not significantlydisrupted by subsequent shaping and which will persist even after thesolid polymer is subsequently removed. The number of near touches in thefibre filled product may be determined by microscopic examination of thenetwork structures is the solid state. The coherence of the structuremay be checked by burning off the polymer matrix and the fibre structureremaining be further examined.

If the basic fibre diameter is of the order of a few microns (as withmonofilaments of glass or steel for example), the required number ofcontacts can be obtained with reinforcement loadings of a few percent byvolume and filament lengths of a few millimetres upwards, according tothe approximate relationship (1) given above.

If some monofilaments are present as bundles then those bundles act asone fibre of relatively low aspect ratio in relationship (1) so far asbuilding a structure is concerned, thereby reducing the effectiveness ofthe fibre material present though not preventing it participating in thestructure.

At loadings of 0.5 to 8 percent based on monofilaments of 3 to 15 mmlength and 8-15 μm diameter the reinforcement works to maximum effecti.e. where mechanical load is applied a very high proportion of thefibres in the load direction will bear load to their natural limit inmarked contrast to constructions where bundles of monofilamentsconstitute the basic fibre elements. In such constructions long lengthsof filaments are weakened below their natural strength limits by beingknotted or twisted round other filaments. This does not mean thatloadings outside the given ranges will not yield substantialreinforcement and other advantages. In fact for a given structure andtest direction relative to it, the tensile strength and stiffness of acomposition according to the invention are broadly proportional to thevolume fraction of fibre up to at least 8% and increase further beyondthis fraction.

If the structure formed within the liquid resin or polymer is to survivethe shaping process it must be deformable without being disrupted. Thisis achieved by systematically bringing fibres (usually filaments) towithin a fibre (usually a filament) diameter or less of each other inthe molten polymer. The force to separate them is then very large butfibres may rotate or slide at their contact points much more easily. Thestructure is then able to conform to the shaping process without beingdisrupted.

The basic repeating pattern of the structure in at least two of threemutually perpendicular directions at a point in the artefact must be ofdimension small compared with the extent of the artefact in thatdirection. This scale relation is crucial to ensuring the extension ofthe structure throughout the artefact and to ensure its uniformity. Bothfeatures are essential for minimising any post-forming distortion in theartefact arising from cooling or later environmental temperaturevariation, and for obtaining a uniform response to mechanical loading.

The invention may be employed for any of the objectives (a) to (e)mentioned above, with products which may be extruded as hollow sectionsor solid profiles or be formed by injection moulding. Thermoplasticsheet extrusions containing fibre structures made according to theinvention may be further shaped by thermoforming under pressure orvacuum to obtain a wide range of artefacts, without significantdisruption of the structure. This is possible because the absence ofsignificant twisting or knotting of the fibre filaments, ensures thatthere is sufficient give in the structure so that filaments can morerelative to each other and to the polymer under such deformation.

Similarly for the same reasons pre-impregnated thermosetting forms maybe further formed under pressure or vacuum to obtain a wide range ofartefacts.

The invention will be further described by way of example and withreference to the accompanying drawings, in which

FIG. 1 defines a co-ordinate axis system for the invention as applied tothe production of sheet,

FIGS. 2a and 2b illustrate one arrangement of flow modifying elementswhich may be used in the apparatus of the invention,

FIGS. 3a-3d illustrate the profile of one form of flow modifyingelement,

FIGS. 4a to 4c illustrate velocity profiles obtained using thearrangement of flow modifying elements in FIGS. 2a and 2b,

FIG. 5 illustrates the orientation of a fibre,

FIGS. 6a-6c illustrate the manner in which fibres are orientated by thesuperimposition of the velocity profiles in FIGS. 4a to 4c,

FIG. 7 illustrates a network structure obtained using the arrangement ofFIG. 2,

FIGS. 8a and 8b show further arrangements of flow modifying elementswhich may be used,

FIGS. 9a and 9b are velocity profiles obtained using the arrangement ofFIG. 8,

FIGS. 10a and 10b show a further arrangement of flow modifying elements,

FIG. 11 illustrates the production of sheet using an apparatus of theinvention,

FIG. 12 illustrates apparatus for producing tubular extrusions,

FIG. 13 illustrates the arrangement of vanes used in the apparatus ofFIG. 12,

FIG. 14 is a section on the line XIV--XIV of FIG. 13, through one ringof vanes shaped for material flowing from the left,

FIG. 15 illustrates the curing of weld lines,

FIGS. 16-18 are photographs of fibre structures obtained using theinvention,

FIGS. 19 and 20 illustrate properties of various fibre reinforcedpolymer products.

The invention will now be described firstly for the production of fibrereinforced sheet followed by hollow sections followed by injectionmoulding. Notwithstanding its primary application to long fibres asdefined above the invention may be applied with advantage to the case ofshort fibres, and this will be referred to where appropriate.

Reference is firstly made to the co-ordinate system of the rectangularsection channel 1 of FIG. 1 which is of a breadth B and a depth d. Thechannel as illustrated is taken to be oriented so that the planesurfaces of the sheet as extruded are horizontal. The x and z axes liein the central horizontal plane of the apparatus with O_(z) in thedirection of extrusion and O_(x) at right angles, i.e. in the transversedirection. O_(y) is normal to the central plane in the upward direction.O_(xyz) thus form a conventional left hand co-ordinate system.

One embodiment of apparatus for use in producing sheet is illustrated inFIGS. 2a and 2b.

FIGS. 2a and 2b illustrate a first configuration of flow modifyingelements in the channel 1. These flow modifying elements (referred to asvanes) are located upstream of an extrusion die (not shown).

The vane configuration shown in FIGS. 2a (which is a view along O_(y)from plane y=-^(d) /₂) and 2b (a view along O_(x)) is referred to hereinas the L-R configuration and comprises a transverse row (a) of aplurality of elongate vanes 2 projecting downwardly from the uppersurface (y=+^(d) /₂) of the channel 1 and arranged (within the row)generally parallel to each other at an angle +n^(o) relative to O_(z). Aplurality of vanes 3 (similar to vanes 2) arranged in a transverse row(b) project upwardly from the lower surface (y=-^(d) /₂) of thechannel 1. Within the row (b) the vanes are parallel to each other andwill generally, but not necessarily, be at an angle -n^(o) relative toO_(z). A plurality of such rows (a) and (b) are provided alternatelyalong the length of the channel typically separated by a distancebroadly of the same order as the vane length. For example a total of (4)of such rows may be provided.

In order to illustrate the shape of a vane 2, reference will now be madeto FIGS. 3(a)-(d) which illustrate a vane for resin moving left to rightas viewed in FIG. 3a. FIG. 3(a) illustrates a portion of a row (a) ofthe apparatus of FIG. 2 showing vanes 2 represented (as straight lines)by the locus of the highest points on the vane. FIGS. 3(b)-(d) arerespectively cross-sections on the lines A--A', B--B' and X--X' of FIG.3a looking in the directions indicated by the arrows. The height h ofthe vane will generally be between one-third and two-thirds (e.g.between one-third and a half) of the depth d of the channel 1 (or widthof annulus for hollow sections). The width (b) of the vane will be asmall fraction of the vane length (l_(v)). The vanes are desirablyshaped to avoid significant weld lines downstream of the vanes. Such aweld line is created by a sufficiently sharp change of geometry in thedie that two neighbouring stream lines diverge with solid surface inbetween them. With purely rectangular vanes such divergence is obtainedat the outer edges. Such divergence may be totally or in very largemeasure avoided by shaping the leading and trailing edges of the vanesso as to have smooth radii in each plane in the light of the elasticproperties of the fibre-bearing polymer. Thus, the right hand end of thevanes as shown in FIG. 3(c) will be a curve which conforms smoothly withthe right hand side face as shown in FIG. 3(b). Similarly, the left handend of the vane FIG. 3(c) will conform smoothly with the left hand sideface (FIG. 3(b). Generally in the light of the elastic properties of thefibre bearing polymer, the upstream approaches U (FIG. 3(d)) to the vanewill be distinctly more gradual than the downstream faces D. This allowsthe streamlines to change direction without interposing solid surfacesbetween two neighbouring stream lines.

Purely by way of example, the dimensions of the vanes illustrated inFIGS. 3(a)-(d) for use in the production of flat sheet of the order of 3mm depth may be as follows:

Height h=1.0 mm

Length l_(v) =6.0 mm

Width b=1.2 mm

The transverse spacing of the vanes within the row, as depicted by x inFIGS. 3(d) may be 5 mm.

Fibre lengths (l_(f)) are chosen to match a number of constraints butdesirably they will be somewhat shorter than vane lengths (l_(v)) butlong enough to meet the criterion for long-fibre behaviour definedabove. This behaviour is also a function of the fibre volume fraction inthe polymer-fibre composition: the shorter the fibre length (l_(f)) thegreater the volume fraction of fibre required to achieve a coherentstructure, as indicated by relationship (1). The invention thus providesflexibility in choosing l_(f) to conform with the geometry of the die, abenefit of particular importance for extruding thin sections of order of1 or 2 mm. Alternatively if the fibre length l_(f) is constrained by therequirements of the product, then the invention provides means fordefining suitable values of h, l_(v) and b.

The fibre structure created by the L-R configuration (FIG. 2) will nowbe described with reference to FIGS. 4a and 4b which show velocityprofiles established in the channel, and FIG. 5 which depicts theorientation of an individual fibre F in the channel. The row (a) of leftdeflecting vanes 2 establishes the velocity profiles sketched in FIGS.4a, 4b and 4c near the outlet from the vane row (FIG. 4a shows the axialvelocity w and FIG. 4b shows the transverse velocity u and FIG. 4c showsthe velocity v). The fibres F will tend to move towards an alignmentangle φ in the horizontal plane defined to a good approximation for theillustrated apparatus by

    tanφ=(∂u/∂y)/(∂w/∂y)(2)

As will be seen from FIGS. 4a and 4b, both velocity profiles (∂u/∂y) and∂w/∂y change sign and magnitude through the depth of the channel. Thusthe angle φ to which a fibre will tend to be orientated will depend onits position in the channel. The effect of the first (L) row of vanes 2gives the initial structure shown in FIGS. 6(a)-(c) which respectivelyshow fibre orientation in the upper, middle and lower thirds of thedepth d of the channel 1. As shown in FIGS. 6(a)-(c) broadly threelayers of fibres F with φ of different sign and magnitude are obtainedemerging from the first L-row.

The three layers are as follows: ##EQU1##

It is in the upper layer that the largest values of φ are obtained andin the lower layers in which φ is smallest.

The desirable vane dimensions given above as proportions of the channeldimensions will generally be sufficient for the orientation defined by(2) to be achieved. The fibres thus have a horizontal orientationdepending on their position within the channel and a structure ofhorizontally overlapping fibres will be built up. If nothing elsehappened there would be little to stop the fibres F realigning to theaxial direction in due course. On emerging from the L-row however,fibres will be tilted towards the vertical O_(y) in varying degreescharacterised by angle θ with the horizontal O_(xz) plane in FIG. 5. Thekey point is that in the space between the L-row and the following R-row(on the bottom face) the fibre vertical angles θ will increase ordecrease at a rate and in a sense broadly determined by the product (forthe case where the vertical velocity component v is small compared withw) ##EQU2##

Thus some but not all fibres F will rotate sufficiently in the spaceavailable--those with the least transverse alignment φ will rotate most.This rotation θ of the fibres results in the ends of some of the fibresbeing inserted between other fibres of high φ and low θ in neighbouringlevels across the depth of the channel. The result is that a networkstructure begins to be built up with the various horizontal levels ofthe fibres being connected by the fibres which have been rotated throughthe angle θ. Such a structure may be regarded as an interlaced network.It is evident from FIGS. 4(a) and 4(b) and the relation for φ thatalignments emerging from the L-row near the lower surface of the die(y=-d/2) will be small compared with those between the horizontal planeand the upper surface (y=d/2) on which the L-vanes are positioned. Aright-ward orienting row of vanes 3 of the lower surface now inducesorientation of the same magnitude as vanes 2 of the L-row but in theopposite sense. Vertical rotation of the least aligned fibres creates asecond weave on emerging from R-row. The velocity profile for the otherperpendicular direction some way away from the exit of a vane, is shownin FIG. 4c. This enhances the rate of rotation θ but the invention isnot dependent on this. The profile increases θ and also helps to ensurethat sufficient fibres enter the next row of vanes with a sufficientangle θ to be slid into the next row.

As well as the rotary motion indicated by relation (3) fibre interlacingis achieved by a sliding motion arising from relative changes in theazimuthal angle φ (FIG. 5). This sliding motion is particularlyimportant for use in channels where as in the example above the depth dmay be less than or of the same order as the fibre length l_(f). Anapproximate relationship for the change of 0 for a typical channel is:##EQU3## which shows that within a distance equal to a typical vanelength l_(v) a minority of fibres (of high θ) will swing almostcompletely to the alignment angle given by relation (2) while those withlow θ will hardly move at all. Combined with the systematic changes in∂u/∂y and ∂w/∂y described above this is precisely what is required toachieve an interlaced or woven structure through the channel depth.

The mean bulk flow direction has thus not been deviated from theextrusion direction over any distance significantly greater than a vanespacing which is designedly small compared with the transverse dimension(or perimeter of a hollow section). A portion of the structure which isbuilt up within the liquid polymer is shown in FIG. 7 (using anarrangement of four rows of vanes) and will be seen to comprise a numberof major strands (the spacing of which is approximately related to thespacing of the vanes) and an "in fill" of monofilaments in alldirections. The overall structure is generally lace-like.

The coherence of the structure depends on the close approach of thesliding or rotated fibres to the majority having substantial orientationφ in the horizontal plane. Subsequent extrusion at the die or flow intoa mould will modify but not disrupt this structure, because theresistance to separating two fibres closer than a fibre diameter is muchgreater than the resistance to their relative rotation.

So far the invention has been described with reference to the L-R vaneconfiguration shown in FIGS. 2(a) and 2(b). Other vane configurationsare however possible, to obtain more compact or more open structureswith greater or lesser degrees of orientation perpendicular to the maindirection of flow.

The vane configuration shown in FIGS. 8a (a view along O_(y) from planey=-d/2) and 8b (a view along O_(x)) is referred to herein as the chevronconfiguration and comprises vanes 4 and 5 provided alternately acrossthe width of channel 1 at angles of +m^(o) and -m^(o) respectively. Allvanes 4 and 5 are on the same surface (y=±d/2) of the channel. The vanes4 and 5 are provided as a transverse row c in the channel, and aplurality of such rows will be provided along the length of the channel.

Velocity profiles established by the chevron configuration are shown inFIGS. 9a and 9b (9a on a section within the chevrons, 9b on a sectiondownstream of a set of chevrons). The structure obtained by use of thechevron configuration is a tighter structure than that obtained with theL-R configuration.

The vane configuration shown in FIGS. 10a (a view along O_(y) from planey=+d/2) and 10b (a view along O_(x)) is referred to herein as the twistconfiguration and comprises vanes 6 provided, at an angle +p^(o)(relative to O_(z)) on the upper surface (y=+d/2) of the channel abovevanes 7 provided at an angle -p^(o) (relative to O_(z)) on the lowersurface (y=-d/2) of the channel. Vanes 6 and 7 are provided in atransverse row (d) of the die and a plurality of such elements areprovided along the length of the channel.

FIG. 11 schematically illustrates the production of a sheet ofthermosetting resin using the vane arrangement of FIG. 10 (twistconfiguration) although the other vane configurations already describedcould equally be used. A mixture of resin and discrete fibres is fedfrom a tank (not shown) in the direction of arrow A into channelcontaining two transverse rows the twist configuration vanes 6 and 7.The sheet 10 containing the network structure emerges from the die lips11 and is collected on a conveyor 12 for further processing asnecessary.

The detailed number, dimensions and disposition of the vanes (2 and 3, 4and 5, 6 and 7) within each row will vary with each application but willdesirably accord with the following principles: the length (l_(v)) of avane will be at least equal to the depth (d) of the die (or in the caseof hollow sections the thickness of the annulus) and usually be a smallmultiple of this dimension: the vanes in elements of the L-R (FIG. 2)and twist configurations (FIG. 10) will be desirably at a uniform angleof typically either +45° or -45° with respect to O_(z) on the top andbottom surfaces of the die (or the inner and outer surfaces of a hollowsection) except that near the edges of a sheet die the vane angles andlengths may be progressively varied somewhat in order to optimise theedge condition. Generally the vanes will be spaced so that looking alongO_(z) in the plane of either the die surfaces (y=+d/2) the projection ofthe vanes overlap somewhat but his need not necessarily be so to obtainbenefit from the invention.

In elements with the Chevron configuration (FIG. 8) the vanes are ofalternate uniform angles typically +45° such that neighbouring vanesapproach each other a minimum distance typically of the order ofone-third a vane length (l_(v)). As with the L-R configuration the row(c) extends uniformly across the die so that both the fibre structurecreated and the flow history of the polymer are fundamentally uniform inthe transverse direction (or in the perimetral direction for hollowsections). Near the edges of the sheet die the vane angles and lengthsmay be varied somewhat to accommodate the solid edge presented by thedie.

The vanes 4 and 5 of the chevron configuration and the vanes 6 and 7 ofthe twist configuration may be of the same profile as illustrated inFIGS. 3(a)-(d).

Reference has been made to the application of the invention to hollowsections. One particular arrangement for use in producing fibrereinforced tube is shown in FIG. 12. This Figure illustrates pipeextrusion apparatus comprising an outer cylindrical housing 20 with aninner cylindrical core 21 supported by spiders (not shown). An annularchannel 101 is defined between housing 20 and core 21 in one section ofwhich are a plurality of flow modifying vanes 102 and 103 (see FIGS. 13and 14), the former of which are provided on the inner surface ofhousing 20 and the latter of which are provided on the outer surface ofcore 21. Downstream of the vanes 102 and 103 is a conventional hotforming die 22. A cold-forming die (not shown) is provided downstream ofdie 22. Heater bands 23 are provided as shown. In use of this apparatus,a fibre/polymer mixture from a conventional extruder head (not shown) isfed to channel 101 for passage through the extrusion apparatus andemerges as pipe with a network reinforcement 24. Referring now to FIGS.13 and 14, a channel 101 is provided with alternating rows of the vanes102 and 103, which may be of the same profile as shown in FIGS.3(a)-(d). As shown in FIG. 13, the vanes 102 (as viewed towards thepolymer flow) are left deflecting (see FIG. 13). The row of vanes 103 isaxially displaced from the row of vanes 102 and, are right deflecting aplurality of rows of vanes 102 and 103 are provided alternately with(i.e. no vane) sections of lengths equal to half a vane length ¹ v alongchannel 101. For example a total of 4 such rows (as illustrated in FIG.12) may be provided. It will be appreciated therefore that the vanearrangements shown in FIGS. 13 and 14 are, in effect, the equivalent inannular form of the L-R vane configurations shown in FIG. 2 for flatsheet.

The production of the fibre reinforcement network within channel 101occurs in a manner entirely analagous to that described for the L-Rconfiguration shown in FIG. 2.

The advantage of the invention by comparison for instance with arotating mandrel system which has been used to orient fibres in thecircumferential (i.e. O_(X)), is three-fold: (a) it applies withoutmodification to non-circular as well as to circular sections, (b) theemerging extrudate has no bulk circumferential motion which in therotating case must be counteracted by the stationary cooling die, (c) nomechanical moving parts are involved.

Conventionally the core or mandrel of a hollow section die may besupported by struts or spiders which join it at typically three pointsto the outer surface. These produce streamline separations in the sensedefined above and the consequent weld line if unassisted will notdisappear or heal in the time before a typical extrudate emerges fromthe die face. In particular, fibres tend not to cross such a weld linethereby amplifying the weakness in the subsequent solid product. Theprocess of first orientating in one plane and then rotating in a planeat right angles described in connection with the L-R configuration hasadvantage in this case. FIG. 15 illustrates the mechanism for achievingthis. Looking into the channel with an L-element on the outer channelsurface, the weld line 105 is initially vertical (FIG. 15 (a)); onemerging from the L-row of vanes the weld line is distorted as shown inFIG. 15 (b). On proceeding downstream, the component of rotation in thevertical (i.e. O_(yz)) plane stitches the fibres F across the weld line.On passing to the R-element the weld line is further diverted (in theopposite direction) and then stitched. The stitching process is assistedby the small component of velocity V (i.e. in the O_(y) direction)immediately downstream of a vane element.

Although the production of tubular extrusions has been described withspecific reference to circular section tubes, the invention is equallyapplicable to profiles generally including square closed and openrectangular sections.

To illustrate the reinforcing network which may be produced in anextruded tube reference is made to FIG. 16 which is a photograph of afibre structure (in this case glass fibre) remaining after the polymermatrix has been removed by burning and shows the essentially isotropicorientation obtained. The fibrous network in this case occupies a volumefraction of 7% within the polymer matrix. An appreciable fraction of thefibres are present as bundles of filaments.

FIG. 17 is a photograph of a lace-like fibre structure obtained at about3% fibre volume obtained using an L-R configuration. FIG. 18 is aphotograph of a fibre structure obtained at about 0.9% fibre volumeusing an L-R configuration where virtually all fibres are present asmonofilaments.

Clearly the different sequences and spacing of vane elements can givedifferent network structures. Generally for relatively open lace-likestructures (FIGS. 17 and 18) the spacing of the vanes will determine onthe average the distance between the main stands of the lace (i.e. theopenness of the structure).

Finally, the invention may with advantage be applied to injectionmoulding particularly where large mouldings (for instance structuralfoam mouldings, injection mouldings, or blow mouldings) demandrelatively large runners prior to the moulding gate. All variants inFIGS. 2, 8 and 10 are applicable. The point here is that gates ofseveral mm, as typically used for such mouldings will permit the passageof the fibre structures created by the vane systems placed in therunners. The advantage is potentially very considerable in that fibrereinforced thermoplastic mouldings normally suffer from fibre alignmentswhich reflect the flow into the mould and not the loading requirementsof the said product. The fibre structures created in the runners willthus greatly enhance the uniformity of the injection moulding.

FIG. 19 shows the process of deformation (FIG. 19(a) before deformationand FIG. 19(b) after deformation) without disruption in a typicalnetwork structure of the invention, e.g. as shown in FIG. 7. Points like200 of the main structure act as hinges predominantly, points 201 act asboth sliders and hinges. The main structure in this case assumes atrellis-like form. Deformation is naturally easier when the polymerbetween the filaments is still molten than when it is solid but therequirement to deform is greatest in the liquid state as the artefact isbeing shaped.

FIGS. 20(a)-20(c) shows how the flow of heat and electricity is affectedby the various categories of fibre structure.

In the structure 20(a) made according to the principles of the inventionthe thermal conductivity of the composition in any direction is broadlyproportional to the thermal conductivity of the fibres (where thisgreatly exceeds that of the polymer) and their volume fraction. Inlaminate structures 20(b), thermal conductivity in the planes of thelaminates 202 is also broadly proportional to the thermal conductivityof the fibres (where this greatly exceeds that of the polymer) and theirvolume fraction, but perpendicular to the laminate plane (i.e. in thedirection Oy in which heat is normally removed for cooling or curingpurposes), the overall thermal conductivity is usually governed by thethermal conductivity of the polymer.

For electrical conduction the position is broadly similar, except thatthe effects are more pronounced when metal fibres are used because ofdifferences of electrical conductivity between metals and polymers aremuch greater than differences of thermal conductivity. In particular,where as in FIG. 20(c) (the short-fibre case) there is no or littlecontinuity of fibre structure in the Oxz plane (i.e. a plane normal tothe paper containing Ox) there will be no screening of incidentelectromagnetic radiation. Both the structure of this invention (FIG.20(a)) and that of the conventional preformed laminates (FIG. 20(b))provide such screening where enough of the fibres actually touch. Theadvantage of the present invention, whereby a structure is achievedwithin the resin immediately prior to shaping, is thus very considerablyin this application also. This is because many of the artefacts forwhich such electromagnetic screening is required are moulded fromthermoplastic compositions for which the use of preformed fibrelaminates 202 is ill-adapted.

I claim:
 1. Polymer resin processing apparatus comprising a channel witha principal flow direction along which fibre containing resin may flowdefined by fixed surfaces, said channel having sets of flow modifyingelements adapted to superimpose a regulated succession of velocityprofiles on the principal flow direction and on the two directionsperpendicular thereto, the flow modifying elements being so dimensionedand orientated and the sets being so positioned relatively to each othersuch that the velocity profiles established by each such set onlypersist over a distance which is small compared to the flow directiondimension of the channel and to the principal transverse dimension ofthe channel and such that resin entering the first set at a particularposition in the cross-section of the channel exits from the last set atsubstantially the same position on the cross-section.
 2. Apparatus asclaimed in claim 1, wherein the flow modifying elements are elongate andare disposed at an angle relative to the principal flow direction alongthe channel, said elements being arranged in a plurality of transverserows along the channel, and each of such rows being comprised of aplurality of the flow modifying elements.
 3. Apparatus as claimed inclaim 2, wherein within each row the flow modifying elements aresubstantially parallel to each other.
 4. Apparatus as claimed in claim3, wherein the transverse rows are provided alternately on oppositefaces of the channel, and the flow modifying elements of one row areangled in the opposite direction to those of the adjacent row on theopposite face of the channel.
 5. Apparatus as claimed in claim 4,wherein the flow modifying elements of the alternate rows are at equalopposite angles to the principal flow direction along the channel. 6.Apparatus as claimed in claim 3, wherein each transverse row of flowmodifying elements on one face of the channel is provided opposite afurther transverse row of flow modifying elements on the other face ofthe channel, the flow modifying elements of such opposed rows are angledin opposite directions to the principal flow direction, and a pluralityof such pairs of opposed rows is provided along the channel. 7.Apparatus as claimed in claim 6, wherein the flow modifying elements ofthe opposed rows are at equal opposite angles to the principal flowdirection along the channel.
 8. Apparatus as claimed in claim 2, whereinwithin any one transverse row the flow modifying elements are arrangedalternately at opposite angles to the principal flow direction, and aplurality of such rows are provided along the channel.
 9. Apparatus asclaimed in claim 8, wherein the alternate flow modifying elements in atransverse row are at equal opposite angles to the principal flowdirection along the channel.
 10. Apparatus as claimed in claim 8,wherein the point of closest approach of adjacent flow modifyingelements in a transverse flow is a minimum of one third of a vanelength.
 11. Apparatus as claimed in claim 2 wherein the flow modifyingelements are at an angle of ±40°-50° relative to the principal flowdirection along the channel.
 12. Apparatus as claimed in claim 11,wherein the flow modifying elements are at an angle of about ±45°relative to the principal flow direction along the channel. 13.Apparatus as claimed in claim 2 wherein the flow modifying elements arevanes projecting into the channel.
 14. Apparatus as claimed in claim 13,wherein the vanes have a height of one third to one half the depth ofthe channel.
 15. Apparatus as claimed in claim 13, wherein the width ofa vane is a small fraction of its length.
 16. Apparatus as claimed inclaim 13 wherein the vanes have smoothly contoured surfaces to preventseparation of fluid flow.
 17. Apparatus as claimed in claim 16, whereinthe upstream face of the vane is of shallower contour than thedownstream face.
 18. Apparatus as claimed in claim 1 wherein the channelis shaped for the production of an article of solid section. 19.Apparatus as claimed in claim 1 wherein the channel is shaped for theproduction of an article of hollow section.
 20. Extrusion apparatuscomprising polymer resin processing apparatus as claimed in claim 1 andan extrusion die located downstream of said processing apparatus in theprincipal flow direction along the channel.
 21. Extrusion apparatus asclaimed in claim 20, having heating means between the downstream flowmodifying element and the extrusion die for establishing a viscositygradient in the polymer so as differentially to increase or reducefibres at least one of the surfaces of the product.
 22. Mouldingapparatus comprising a polymer resin processing apparatus as claimed inclaim 1 and a mould cavity located downstream of said processingapparatus in the principal flow direction along the channel.
 23. Aprocess for the preparation of a fibre filled liquid polymer compositioncomprisingpassing liquid resin containing discrete fibers along a flowchannel adapted to impose on the resin a regulated succession ofvelocity profiles on the principal flow direction and on the other twodirections perpendicular thereto, the individual profiles persistingover distances which are small compared to the principal flow directiondimension of the channel and to the principal transverse dimension ofthe channel, the profiles being such that resin at a particular positionon the cross-section of the channel where the first of the sequencevelocity profiles is imposed is at substantially the same position onthe cross-section of the channel immediately after the last of thesequence of velocity profiles has been superimposed and such that themutually perpendicular velocity profiles superimpose on each other so asto result in at least rotation or sliding of the fibers relative to theresin such that as the resin passes along the channel the fibers buildup a network structure.
 24. A process as claimed in claim 23, whereinthe fibres are monofilaments.
 25. A process as claimed in claim 23wherein the fibres have a length of 5 to 15 mm.
 26. A process as claimedin claim 23 wherein the fibres are glass fibres, organic fibres, ceramicfibres or metal fibres.
 27. A process as claimed in claim 23 wherein theresin is a thermoplastic.
 28. A method of extrusion comprising preparinga fibre filled resin composition using the process of claim 23 andextruding the composition through a die.
 29. A method of mouldingcomprising preparing a fibre filled resin composition using the processof claim 23 and introducing the composition into a mould cavity.
 30. Afiber filled polymer product comprising a polymer matrix containing anetwork of discrete fibers in which the orientation of the fibers issubstantially independent of the axis along which resin has flowedduring manufacture of the product, the network being comprised of majorstrands of a plurality of filaments and an in-fill of mainly singlefilaments in spaces bounded by the major strands wherein the averagenumber of near touches which each fiber makes with adjacent fibers isequal to N where N is in the range 0.5×c (l/d) to 2.5×c (l/d) with aminimum value of 8, where c is the volume fraction of the fibers in theproduct and is in the range above 0.005 to 0.1 for a thermoplastic andin the range above 0.005 to 0.4 for a thermosetting resin, and l/d isthe aspect ratio of the fibers.
 31. A product as claimed in claim 30,wherein c is greater than 0.005 to 0.08.
 32. A product as claimed inclaim 31, wherein c is greater than 0.005 to 0.05.
 33. A product asclaimed in claim 30 wherein the polymer is a thermoplastic.
 34. Aproduct as claimed in claim 30 wherein the fibres are monofilaments. 35.A product as claimed in claim 30 wherein the fibres have a length of 3to 15 mm.
 36. A product as claimed in claim 30 in which no specificfibre matrix coupling agents are used.
 37. A product as claimed in claim30 which is a hollow extrudate.
 38. A product as claimed in claim 30wherein the fibres are electrically conducting and the network structurescreens the passage of electromagnetic waves through the material.
 39. Aproduct as claimed in claim 30 wherein the fibres have at least 10 timesthe conductivity of the polymer, and the network structure increases thethermal conductivity of the product by at least one third×c×thermalconductivity of the fibres.
 40. A product as claimed in claim 30 whereinN is in the range 0.8×c (l/d) to 2.5×c (l/d).